Retrofocus lens system



United States Patent lnventor Tomokazu Kazamaki Tokyo-to, Japan Appl.No. 648,909 Filed June 26, 1967 Patented Nov. 10, 1970 Assignee AsahiKogaku Kogyo Kabushiki Kaisha Tokyo-to, Japan a corporation of JapanPriority June 30, 1966 Japan ll/42,823

RETROFOCUS LENS SYSTEM 2 Claims, 2 Drawing Figs.

US. Cl 350/2l5, 350/207, 350/234 Int. Cl. G02b 9/00, G02b 9/62, G02b11/34 Field of Search 350/207, 183, 215, 234

References Cited UNITED STATES PATENTS 7/1953 Klein ..350/2l5(X)UXAssistant Examiner-Paul A. Sacher Attorney-Stanley Wolder ABSTRACT: Acompact retrofocus wide angle lens system comprises twelve lenses, thefirst three lenses being positive, negative and negative respectively,the next three lenses being cemented as a unit and being successivelynegative, positive and negative, the next three lenses being cemented asa unit and being successively positive, negative and positive and thelast three lenses being cemented as a unit and being successivelypositive, negative and positive. The front face of the fourth lens isflat or concave, the Abbe number of the fourth lens is greater than thatof the fifth lens, the refractive index of the fifth lens is greaterthan that of the sixth lens, the index of refraction of the eighth lensis greater than that of the seventh lens and of the ninth lens and therefractive index of the eleventh lens is greater than that of the tenthlens and that of the twelfth lens.

Patented Nov. 10, 1970, l V v 3,539,246

T h "lNvENTgR E 0 o Oil-IA Q m ATTORNEY Patented Nov. 10, 1970 Sheet 1'.0 n' 4.0 o 1.0 SPl-ERICAL ABERRATION DISTORTION SINE CONDITION (SC)ASTIGMATISM INVENTOR TOMOKAZU KAZAMAKI ATTORNEY RETROFOCUS LENS SYSTEMBACKGROUND OF THE INVENTION The present invention relates generally toimprovements in lens systems and it relates particularly to an improvedretrofocus wide angle lens system.

In retrofocus lens systems of conventional design, the diameter of thefront or first lens increases as the length of the back focus increasesand the angle of the field becomes greater. Particularly in a lenssystem having a wide angle of field, such as the Flektogon f 20 mm. 1:4,the diameter of the front lens is about 55 mm. and the overall length ofthe lens system is very large. As a consequence, conventional lenssystems of this type are heavy and bulky so that cameras provided withthese lens systems are difficult and awkward to handle and asatisfactory steady hand support and manipulation thereof is almostimpossible. Moreover, a carrying case for a camera provided with theconventional wide angle retrofocus lens system, in order to accommodatethe lens system is correspondingly awkward and bulky and expensive.

SUMMARY OF THE INVENTION It is a principal object of the presentinvention to provide an improved wide angle lens system.

Another object of the present invention is to provide an improvedretrofocus wide angle lens system.

Still another object of the present invention is to provide an improvedhighly compact retrofocus wide angle lens system.

A further object of the present invention is to provide an improvedcompact retrofocus wide angle lens system of high optical quality.

The above and further objects of the present invention will becomeapparent from a reading of the following description taken inconjunction with the accompanying drawing which illustrates a preferredembodiment thereof.

In a sense, the present invention contemplates the provision of aretrofocus lens system comprising twelve lenses consecutively designatedas the first to the twelfth and'satisfying the following conditions:

wherein r is the radius of curvature of the front face of the fourthlens, 11, is the Abbe value of the subscript designated lens, and n, isthe index of refraction of the subscript designated lens.

The first lens is positive, and each of the second and the third lensesis of the negative meni-type with the surface of larger curvaturedirected to the image side. The fourth, the fifth and the sixth lensesare cemented together and resultant lens group is so placed as to be asclose to the third lens as possible. The fourth lens is negative, thefifth lens is positive and the sixth lens is of the negative meniscustype with the surface of the larger curvature directed to the objectside. The seventh-lens is positive, the eighth lens is negative and the'ninth lens is positive, these three lenses being cemented together. Thetenth lens is positive, the eleventh lens is negative and the twelfthlens is positive, these three lenses being also cemented together. Adiaphragm is positioned between the ninth and the tenth lenses.

The resultant power of the first, the second and the third lenses,designated as F is of negative value, and in order to lengthen the backfocus of a retrofocus lens system, it is generally necessary to increasethe absolute value of f In lens system designing practice, however, anyexcessive increase in the negative power of f must be avoided since itwould result in an excessive overcorrection of Petzval sum. Thegenerally practiced method is to utilize a certain smaller value of fand to provide a larger spacing from the next positive lens, thuslengthening the back focus. With such method, however, it is unavoidablethat the first lens becomes excessively large as the angle of fieldincreases.

In accordance with the present invention, an optical element is placedat the region corresponding to said air spacing so as to prevent thefirst lens from becoming larger. This optical element corresponds to thethick cemented-lens element consisting of the fourth, the fifth and thesixth lenses. An important and unique feature of the present lens systemis that the radius of curvature r, of the front face of the fourth lensis of negative value or infinite, r 0, that is, the fourth lens frontface. is flat or concave.

With the condition 11,, 11 if a greater Abbe number 11, of the firstlens is not employed, the chromatic difference of magnificationaggravates so that the cementation is adopted to eliminate color asearly as possible. The condition n n provides a suitable correction ofcoma aberration in the wide angle of the field of the present lenssystem. The conditions n n n n n and n n prevent the overcorrection ofthe Petzval sum.

BRIEF DESCRIPTION OF THE 'DRAWINGS FIG. 1 is a schematic representationofa preferred embodiment ofthe invention.

FIG. 2 illustrates the aberration curves of the preferred embodiment.

DESCRIPTION OF PREFERRED EMBODIMENT Referring now to the drawing andparticularly FIG. 1 thereof which illustrates a preferred embodiment ofthe present invention, the improved lens system comprises twelvecoaxially positioned lenses designated'successively as lenses L1 to L12respectively. The fourth, fifth and sixth lense s L4, L5 and L6 arecemented together with their confronting faces in mating registry toform a unit, the seventh, eighth and ninth lenses L7, L8 and L9 arecemented together with their confronting faces in mating registry toform a unit, and the tenth, eleventh and twelfth lenses L10, L11 and L12are cemented together with their confronting faces in mating registry toform a unit.

The first lens, L1 is a positive lens with an index of refraction n anda thickness d,, a front face of radius of curvature r and a rear face ofradius of curvature r the second lens L2 is a meniscus type negativelens spaced from the lens L1 a distance s and having a thickness d arefractive index n a convex front face of radius of curvature r and aconcave rear face of radius of curvature r.,; and the third lens L3 is ameniscus type negative lens spaced from the lens L2 a distance s andhaving a thickness d a refractive index n;,, a convex front face ofradius of curvature r and a concave rear face of radius of curvature rThe fourth, fifth and sixth lenses, L4, L5 and L6, are a cementedtriplet unit, the lens L4 being negative and spaced from the lens L3 adistance s and having a thickness d.,, a refractive index n,, a frontface of radius of curvature r-, and a rear face of radius of curvature rthe fifth lens L5 being a positive lens of thickness d with a front faceof radius of curvature r and a rear face of radius of curvature r,,; thesixth lens L6 being a negative meniscus type lens with its rear faceconvex and having a thickness d a refractive index n,,, a front face ofradius of curvature r and a rear face of radius of curvature r Theseventh, eighth and ninth lenses, L7, L8 and L9, are cemented togetheras a triplet unit, the seventh lens L7 being positive and spaced fromthe lens L6*a distance s and having a thickness d,, an index ofrefraction n-,, a front face of radius of curvature r and a rear face ofradius of curvature r,,; the

eighth lens L8 being negative and having a thickness d an index ofrefraction n a front face of radius of curvature r and a rear face ofradius of curvature r and the ninth lens L9 being positive and having athickness d.,, an index refraction n a front face of radius of curvaturer and a rear face of radius of curvature r The tenth, eleventh andtwelfth lenses, L10, L11 and L12, are cemented together as a tripletunit, the tenth lens L10 being positive and spaced from the lens L9 adistance s and having a thickness d an index of refraction n a frontface of radius of curvature r and a rear face of radius of curvature rthe eleventh lens L11 being negative and having a thickness d arefractive index n,,, a front face of radius of curvature r and a rearface of radius of curvature r and the twelfth lens L12 being positiveand having a thickness d refractive index n a front face of radius ofcurvature r, and a rear face of radius of curvature r A centrallyapertured diaphragm D is coaxially positioned between the ninth andtenth lenses L9 and L10 respectively. It should be noted that the lensspacings are measured axially between the confronting faces of thecorrespondingly spaced lenses. The dimensions, relationships andparameters of the lenses are such as to satisfy the conditionspreviously set forth.

The following Table I recites the dimensions and parameters of aspecific example of the present lens system having a focal length ofF lmm.

TABLE I.LENS DATA Table 11 sets forth the Seidel coefficients of thespecific lens system when F 1 mm. and the diaphragm D is positionedbetween the ninth and tenth lenses.

TABLE II.-SEIDEL COEFFICIENTS S S S; P S

1 0. 066 0. 051 0. 272 0. 252 2 0. 008 0. 142 0. 093 0. 845 3 -0. 012 0.050 0. 216 0. 674 4 1. 119 -0. 139 -0. 934 0. 134 5 0. 506 0. 076 0. 4430. 078 6 2. 321 0. 176 0. 933 0. 084 7 0. 700 0. 362 -0. 166 0. 101 8 0.081 0. 002 0. 023 -0. 000 0 0. 146 0. 025 0. 020 0. 007 10. 0. 088 0.248 0. 201 0. 131 11 3. 148 0. 177 0. 615 0. 044 12. 0. 004 0. 002 0.020 0. 015 13.. --1. 043 -0. 038 -0. 198 0. 008 14 t --1. 902 0. 468 0.283 -0. 185 15 i 1. 004 0. 317 0. 193 1. 161 16 O. 508 0. 018 -0. 096 0.004 17 3. 285 -0. 402 -0. 116 0. 063 18 -2. 081 0. 093 0. 634 0. 032

In table I, r, are the radii of curvature of the subscript designatedlens faces and d, the lens spacings and lens thicknesses as identifiedabove, the numerators of n are the indices of refraction of thecorresponding subscript designated lenses and the denominators are therespective Abbe numbers thereof.

As seen in FIG. 2 of the drawings, which illustrate the aberrationcurves of the specific lens system with F l00 mm., although the angle offield is wide, the Petzval sum and astigmatisms are small and with ahalf-angle of field of 47.92, the diameter of the first lens is lessthan 1.4 F which in lens systems of the subject type, is extremelysmall.

While there has been described and illustrated a preferred embodiment ofthe present invention, it is apparent that numerous alterations may bemade without departing from the spirit thereof.

lclaim:

1. A retrofocus lens system comprising twelve lenses consecutivelydesignated as the first to the twelfth lens, the fourth, fifth and sixthlenses constituting a cemented triplet, the seventh, eighth and ninthlenses constituting a cemented triplet and the tenth, eleventh andtwelfth lenses constituting a cemented triplet, the lens faces beingconsecutively designated as the first to eighteenth with each pair ofconfronting mating surfaces of said triplets defining a single lensface, said lenses having the following dimensions and parameters:

wherein r, is the radius of curvature of the i-th lens face, d, is

the thickness of the n-th lens, s, is the axial spacing being the n-thlens and the next successive lens, n, is the index of refraction andAbbe value of the n-th lens.

2. A retrofocus lens system comprising twelve lenses designated as thefirst to the twelfth lens, said first lens being positive and saidsecond and third lenses being negative meniscus lenses with their rearfaces being concave, said fourth, fifth and sixth lenses being acemented triplet unit, the fourth lens being negative, the fifth lensbeing positive and the sixth lens being a negative meniscus lens withits rear face convex, said seventh, eighth and ninth lenses being acemented unit, the seventh lens being positive, the eighth lens beingnegative and the ninth lens being positive, and said tenth,

eleventh and twelfth lenses being a cemented unit, said tenth lens beingpositive, said eleventh lens being negative and said twelfth lens beingpositive, the curvature of the front face of the fourth lens beingnonconvex, the Abbe value of the fourth lens being greater than that ofthe fifth lens, the index of refraction of the fifth lens being greaterthan that of the sixth lens, the index of refraction of the eighth lensbeing greater than those of the seventh and ninth lenses and the indexof refraction of the eleventh lens being greater than those of the tenthand twelfth lenses.

